ATLANTA (AP) — The second American aid worker recently diagnosed with Ebola in west Africa landed via chartered plane in the Atlanta area Tuesday morning, three days after the first American patient's arrival, and was headed to the same specialized isolation unit for treatment.

The second patient arrived at Dobbins Air Reserve Base in Marietta, Georgia, late Tuesday morning in a plane from Monrovia, Liberia's capital, and was headed to Emory University Hospital.

Although hospital officials haven't released the patient's identity, officials from SIM USA — the aid group for which she was working — have identified her as Nancy Writebol. Writebol remains in serious but stable condition, SIM said in a statement Tuesday morning.

Writebol had been in isolation at her home in Liberia since she was diagnosed last month and is able to walk with assistance, according to SIM President Bruce Johnson.

Writebol, 59, and Dr. Kent Brantly, 33, a physician with North Carolina-based Samaritan's Purse, have been described as critically ill after treating Ebola patients at a missionary clinic in Liberia. The country is one of four West African nations dealing with the world's largest Ebola outbreak that has killed at least 887 people in four West African countries.

Ebola is spread by close contact with blood and other bodily fluids. At the clinic, Writebol's duties included disinfecting doctors and nurses entering or leaving the Ebola treatment area. Her son, Jeremy Writebol, has said his parents have spent the past 15 years doing international humanitarian work, and he hopes the attention focused on her case "might help develop a cure and resources to help those who are suffering."

There is no cure for the virus, which causes hemorrhagic fever that kills as many as 60 percent to 80 percent of the people it infects in Africa. Africa's under-developed health care system and inadequate infection controls make it easier for the Ebola virus to spread and harder to treat.

Brantly arrived in Atlanta for treatment Saturday and was the first person infected with the virus to be brought to the United States from Africa. He and Writebol have been given doses of an experimental treatment since they were diagnosed with the virus.

The experimental treatment is made by Mapp Biopharmaceutical of San Diego, with funding from the government. The treatment is aimed at boosting the immune system's efforts to fight off the virus. It is made from antibodies produced by lab animals exposed to parts of the Ebola virus. It's unclear whether the experimental treatment played in recent improvement in the aid workers' medical conditions.

The isolated unit at Emory where Brantly and Writebol will be treated is one of four of its kind in the country and was built in partnership with the Centers for Disease Control and Prevention, hospital officials have said. The unit was built to house patients suffering from serious infectious diseases and in 2005 handled patients with SARS, a viral respiratory illness that spreads when carriers cough or sneeze.

Writebol and Brantly will be sealed off from anyone who isn't wearing protective gear. Dr. Bruce Ribner, who will be treating Brantly and Writebol, has said a plate glass window equipped with a communication system will separate the aid workers from their relatives.

Officials at the Atlanta-based CDC have said they've gotten some criticism for bringing Ebola cases to an American hospital. But CDC Director Dr. Tom Frieden emphasized recently that there is no threat to the public in the United States.